The mechanism which coordinates the replication and the segregation of the genetic material is one of the most fundamental examples of a system of metabolic regulation. Since the processes of chromosome reproduction and mitosis in higher organisms undoubtedly proceed by highly complex and elaborate mechanisms which have to be coordinated, the analysis of these mechanisms and the structural components participating therein will almost certainly prove to be a very difficult problem. We propose to investigate the nature of the control of DNA synthesis at a more elementary level in the bacteria. In particular we will study a system consisting of several independently replicating genetic determinants, the host chromosome and one or more bacterial episomes, which can be considered to be a prototype of the more complex chromosomal systems of higher organisms. There will be four major areas of investigation: The control of the replication of autonomous genetic elements, the structure of nonreplicating and replicating episomes, the mechanism of segregation of genetic determinants at cellular division, and the localization and the internal organization of the genetic material in bacteria. In addition, we will begin a new series of investigations on the isolation and characterization of proteins which bind to episome DNA. These problems will be investigated using a variety of physical, biochemical and genetic techniques. After the completion of these studies, we intend to utilize the information obtained at this level of organization to formulate more precise questions on the molecular events accompanying chromosome reproduction and mitosis in higher organisms.